Polypropylene fiber and cement were used to modify iron tailings and applying it to roadbed engineering is an important way to promote the sustainable development of the mining industry. However, the existing studies are mostly concerned with the static mechanical properties, and lack the deformation characteristics of cyclic loading under different loading modes. The effects of fiber content, dynamic-static ratio (Rcr) and curing age on the deformation characteristics of fiber cement modified iron tailing (FCIT) under different cyclic loading modes were explored through dynamic triaxial tests. The research results show that: (1) Polypropylene fibers significantly reduced the cumulative strain of FCIT. Under intermittent loading, the cumulative strain decreased by 36 similar to 43 %, and under continuous loading, the cumulative strain decreased by 48 similar to 55 %. (2) The deformation behavior of FCIT under both intermittent and progressive loading was in a plastic steady state with cumulative strain <= 1 %. (3) The cumulative strain variation of FCIT with intermittent loading of 0.316 % was significantly lower than that with continuous loading of 0.417 %, and the resilience modulus was higher with intermittent loading. (4) The stress history effect of step-by-step loading can be eliminated by the translational superposition method, and the strain evolution law under continuous loading is predicted based on the progressive loading data, and the minimum error between the expected and actual results is 6.5 % when Rcr is 0.1.

In previous train operations, traffic loads were typically considered continuous, disregarding the intermittent effects of successive trains on subgrade loess. To investigate the cumulative plastic strain behavior and critical dynamic stress of subgrade loess under intermittent train loads, a series of dynamic triaxial tests were conducted considering factors such as cyclic stress ratio, confining pressure, and frequency. The deformation characteristics of subgrade soil under different stress levels were analyzed, and the dynamic behavior of specimens was categorized based on the development trends of strain rate and cumulative plastic strain. Then the critical dynamic stress levels for plastic shakedown and plastic creep states were determined. The results indicate that intermittent effects suppress the development of cumulative plastic strain and excess pore water pressure in the soil. The more cycles of the unloading-drainage stage the soil undergoes, the stronger its resistance to failure. Under intermittent loads, cumulative plastic strain increases with higher cyclic stress ratios and frequencies. When the cyclic stress ratio is constant, the increase in confining pressure enhances soil stiffness, but this increase is insufficient to counteract the strain induced by greater dynamic stress amplitude, resulting in increased cumulative strain. Combining cumulative plastic strain and plastic strain rate, a classification standard for the deformation behavior of subgrade loess under intermittent loading conditions was established, and the critical dynamic stress was identified. The critical dynamic stress increases with higher confining pressure but decreases with frequency. Accordingly, empirical formulas for critical dynamic stress concerning confining pressure and frequency were proposed. These findings are crucial for understanding the mechanism of intermittent train load effects and analyzing subgrade settlement.

Existing ballasted track subgrades are prone to complex particle migration problems due to intermittent train load-rainfall wetting coupling, which causes mud pumping in severe cases. In this work, a model test on a ballast layer overlying a fine particle layer was conducted under intermittent load-wetting coupling conditions. The experimental results indicate that the coupling effect of intermittent loading and wetting has a significant effect on the increase in the volumetric water content and pore water pressure. The changes in the accumulated deformation, resilient modulus, damping ratio, and particle migration phenomenon mainly occur in the first three loading stages (LS1-LS3 stages), and the changes are most significant in the second loading stage (LS2 stage) because of the high saturation and low density of the soils. During the subsequent loading stages, the changes in the accumulated deformation, resilient modulus, damping ratio, and particle migration phenomenon are not obvious because of the high density of the soils. A low level of resilience occurs during intermittent periods (IS4-IS7). At the end of the test, the ballast fouling index (FI) was 16.4%, reaching a moderate fouling level. Timely replacement and rectification should be conducted for sections that produce mud pumping and ballast fouling.

Karst collapse as a unique environmental geological hazard in karst areas, easily causes changes in surrounding water and soil environments. Train-induced vibration is a significant inducement for shallow karst ground collapse. Previous studies on the dynamic properties of surrounding soil under train vibration loads often neglected the impact of time intermittent effects. Taking the red soil covering a typical potential karst collapse area along a high-speed railway in China as the research object, field monitoring of the vibration characteristics of the surrounding environment was conducted. A series of continuous loading and continuous-stop-continuous dynamic triaxial tests and scanning electron microscopy (SEM) tests were designed considering factors such as loading frequency, intermittent duration, and dynamic stress amplitude. The effects of loading intermittence on the dynamic response and microstructure of red soil were compared and analyzed. The experimental results show that the drainage and unloading of red soil samples during the intermittent phase dissipate the accumulated excess pore water pressure and adjust the internal particle and structure of the soil, reducing the accumulation of plastic deformation during subsequent loading stages. The residual strain under vibration loading conditions considering the time intermittent effect is significantly reduced, and the residual strain decreases significantly with the increase of time intervals. The weakening effects of both macro and micro characteristics of red soil in karst-prone areas are significantly enhanced with the increase of intermittent time. The research results are of great significance for the prevention and control of karst ground collapse in karst areas.

In practical engineering applications, cured lightweight soils are commonly used as roadbed fillers and subjected to intermittent and discontinuous traffic loads. However, previous studies primarily focused on the effects of continuous loading on the mechanical properties of cured soils. To address this knowledge gap, this study investigated the deformation characteristics of fiber-reinforced cured lightweight soils under dry and wet cycles and intermittent loading. Dynamic triaxial tests with varying intermittent ratios and numbers of dry and wet cycles were conducted to assess the influence of these factors on the accumulated plastic strain of fiber-reinforced cured lightweight soils. Based on the test results, a prediction model was developed to estimate the accumulated plastic strain of the cured soils under intermittent loading. The findings indicated that the interval length has a dampening effect on the accumulated plastic deformation of the soil, thereby improving its ability to resist deformation. Additionally, the accumulation of plastic deformation gradually increased with the number of wet and dry cycles but eventually stabilized. In multistage loading, the accumulated plastic strain displayed a rapid increase and stabilization trend similar to that in observed the first loading stage. However, the magnitude of the cyclic dynamic stress ratio determines the deformation at later loading stages. Finally, an improved exponential model was used to establish and validate a prediction model for the cumulative plastic strain of the fiber-reinforced cured lightweight soil under intermittent loading (single and multistage). This prediction model provides important guidance for the practical application of fiber-reinforced cured lightweight soils in engineering projects.